The present invention relates to a motor which is suitable for application as the motor for driving a fan for a room air conditioner or the like, and, more particularly, to a brushless motor incorporating an inverter which enables variable speed control.
Recently, as shown in "Japan Radio Newspaper" on Mar. 22, 1990, a high-voltage single chip three-phase inverter (hereinafter referred to as a one-chip three-phase inverter) has been developed which controls the speed of a motor by utilizing a direct power source voltage that is obtained by rectifying and smoothing a commercial AC voltage of 100 (V). The one-chip three-phase inverter is extremely small as compared with the conventional inverter and can be housed in a motor.
As shown in FIG. 1A, this three-phase one-chip inverter has a structure such that polysilicon is used as the base or substrate, and the area of each phase inverter portion is formed in a respectively separate region and is isolated by a dielectric layer, such as SiO.sub.2, and has a high withstand voltage.
The circuits in the two-dotted line of FIG. 2 are integrated on the one-chip.
FIG. 1B is a plan view showing the layout of each element of the one-chip three-phase inverter. As is clear from this diagram, this one-chip three-phase inverter comprises, in a single chip integrated circuit, six switching transistors 2 as main elements, a diode 1 for turning off the switching transistors 2 connected between the collector and the emitter of each of the switching transistors 2, a logic circuit 6 for forming a switching signal for turning on and off each of the switching transistors 2, a drive circuit 3 for on- and off- driving each switching transistor with this switching signal, an excess current protection circuit 5 for preventing the integrated circuit from being destroyed by an excess current, by detecting a current that flows through the switching transistors 2, and an internal power source 4. The integrated circuit element of the one-chip three-phase inverter has the dimensions of a 4.3 mm length and a 5.8 mm width.
In the one-chip three-phase inverter explained above, a lateral Insulated Gate Bipolar Transistor (IGBT) has been developed and employed as the switching transistor 2 to have a substantially reduced area as compared with the conventional power MOSFET. At the same time, a high-speed diode has been newly developed and employed that can be prepared in the same process as that of the lateral IGBT, so that an inverse recovery current is reduced substantially thereby to reduce substantially the switching loss of the switching transistors 2 due to inverse recovery current. Further, a power source circuit is incorporated in the one-chip three-phase inverter to require only one external power source for driving the switching transistors 2 that form the power element. The excess current protection circuit 5 is also incorporated in the one-chip three-phase inverter to prevent the integrated circuit from being destroyed by an excess current that is generated by short-circuiting of a load or the like. Further, the inverter frequency is set to 20 KHz which is higher than an audio frequency so that noise of the motor can be reduced substantially.
FIG. 2 is a block diagram showing a brushless motor that uses the one-chip three-phase inverter as described above. 7A, 7B and 7C designate Hall device sensors, 8 a sensor amplifier, 9 a speed signal formation circuit, 10 a speed correction circuit, 11 a pulse width modulation (PWM) signal formation circuit, 12 a start current limit circuit, 13 an oscillation circuit, 14 a stator, 15 the above-described one-chip three-phase inverter, and 16 an external power source.
Referring to FIG. 2, when a commercial AC voltage of 100 (V) is applied to the external power source 16, a DC power source voltage (VI) is applied to each circuit from the external power source 16. By applying a voltage (VZ) for a signal, the oscillation circuit 13 is started. When a speed command is given to the speed correction circuit 10 from an external circuit, the drive circuit 3 on- and off- drives each of the switching transistors 2 sequentially according to signals generated from the logic circuit 6. Then, a current flows to each coil provided in the stator 14 in a predetermined direction, so that a rotor; not shown, starts rotation to start the motor.
When the motor is started, the start current limit circuit 12 controls the PWM signal formation circuit 11 based on the result of detection by the excess current protection circuit 5 so that the start current flowing through each switching transistor 2 is not excessive, thereby to adjust the duty ratio of the PWM signal.
Assume that the switching transistors 2 are expressed as Q.sub.1, Q.sub.2, Q.sub.3, Q.sub.4, Q.sub.5 and Q.sub.6 and the diodes 1 to be connected to these switching transistors 2 are expressed as D.sub.1, D.sub.2, D.sub.3, D.sub.4, D.sub.5 and D.sub.6. Then, the collectors of the switching transistors Q.sub.1 to Q.sub.3 are connected to the + terminal of the external power source 16 and the emitters of the switching transistors Q.sub.4 to Q.sub.6 are connected to the terminal of the external power source 16. The emitter of the switching transistor Q.sub.1 and the collector of the switching transistor Q.sub.4 are connected to the first coil provided in the stator 14. Similarly, the emitter of the switching transistor Q.sub.2 and the collector of the switching transistor Q.sub.5 are connected to the second coil, and the emitter of the switching transistor Q.sub.3 and the collector of the switching transistor Q.sub.6 are connected to the third coil.
The drive circuit 3 turns on the switching transistors Q.sub.1, Q.sub.2 and Q.sub.3 sequentially at an electrical angle of every 120 degrees, and turns on the switching transistors Q.sub.4, Q.sub.5 and Q.sub.6 sequentially by chopping with a PWM signal at an electrical angle of every 120 degrees. The driving timings of the switching transistors Q.sub.1 to Q.sub.6 are shown as Q.sub.1 to Q.sub.6 in FIG. 3. In FIG. 3, the switching transistor Q.sub.4 is turned on and off at the same cycle and the same duty ratio as those of the PWM signal during the period from the latter half of the on-period of the switching transistor Q.sub.2 to the former half of the on-period of the switching transistor Q.sub.3, the switching transistor Q.sub.5 is turned on and off at the same cycle and the same duty ratio as those of the PWM signal during the period from the latter half of the on-period of the switching transistor Q.sub.3 to the former half of the on-period of the switching transistor Q.sub.1, and the switching transistor Q.sub.6 is turned on and off at the same cycle and the same duty ratio as those of the PWM signal during the period from the latter half of the on-period of the switching transistor Q.sub.1 to the former half of the on-period of the switching transistor Q.sub.2.
As described above, when the motor starts, the Hall device sensors 7A, 7B and 7C detect the rotation of the rotor, and generate rotor position signals of an electrical angle of 180-degree width, with phase differences of an electrical angle 120 degrees at a rotor rotation electrical angle of 360 degrees. These rotor position signals are amplified and waveform-shaped by the sensor amplifier 8 which has been adjusted to a predetermined gain. These signals are then supplied to the logic circuit 6 of the one-chip three-phase inverter 15, and one of the rotor position signals, for example the rotor position signal generated in the Hall device sensor 7A, is supplied to the speed signal formation circuit 9 so that a speed signal representing the speed of the rotor is formed by the frequency or cycle of the rotor position signal. The speed signals are supplied to the speed correction circuit 10 and compared with the speed command supplied from the outside, so that a speed correction signal is formed in accordance with the difference between the signals. The duty ratio of the PWM signal outputted from the PWM signal formation circuit 11 is controlled by the speed correction signal.
In order to improve the precision of speed detection, three speed signals obtained from the three Hall device sensors can be inputted into the speed signal formation circuit 9.
The logic circuit 6 forms a commutation signal (switching signal) for sequentially turning on and off the switching transistors Q.sub.1 to Q.sub.3 with an electrical angle 120 degrees and a deviated phase of every 120 degrees from the three-phase rotor position signals supplied from the sensor amplifier 8, and a switching signal having the same cycle and the same duty ratio as those of the PWM signal supplied from the PWM signal formation circuit 11 with the timing explained in FIG. 3 in accordance with this commutation signal, and sends these signals to the drive circuit 3.
By the above arrangement, a current conduction time for each coil provided in the stater is controlled in accordance with the duty ratio of the PWM signal that has been corrected by the speed correction signal from the speed correction circuit 10, and the rotor speed is controlled to be consistent with the external speed command. When the rotor speed changes, the output cycles of the Hall device sensors 7A, 7B and 7C also change accordingly.
As described above, the rotor speed is determined by the duty ratio of the PWM signal, and the speed of the motor can be changed by changing the duty ratio.
FIG. 4 is an exploded perspective view showing one example of a conventional motor incorporating a one-chip three-phase inverter, based on the circuit structure shown in FIG. 2. 17 designates an upper case, 18 a stator core, 19 a coil, 20 an aperture, 21A, 21B and 21C supporting members, 22 a shaft, 23A and 23B bearings, 24 a rotor, 25 a printed wiring panel, 26 a peripheral circuit, 27A, 27B and 27C bolts, 28 lead wires, 29 a lower case, 30A, 30B, 30C and 30D openings, 31 a drawing duct, 32 an opening, 33 a bolt and 34A, 34B and 34C bolt holes.
Referring to FIG. 4, the cylindrical stator core 18 having the coil 19 wound up in an internal slot is engaged inside the upper case 17. The opening 20 is provided at the center of the upper surface of the upper case 17, and an edge portion is formed at the lower end of the external periphery of the opening 20. The four bolt holes 34A, 34B, 34C and 34d (the last of which is not shown because it is located at the rear side) are provided at equal intervals around the edge portion. The bar-shaped supporting members 21A, 21B and 21C which stretch downward are provided around equal intervals at the external periphery of the lower surface of the stator core 18.
The rotor 24 is covered with a ferrite magnetic material having a thickness of about 2 mm on the peripheral surface, and the shaft 22, passing through the center of the rotor 24, is integrated. The bearings 23A are fixed to the shaft 22 above the rotor 24, and the bearings 23B are fixed to the lower end of the shaft 22. The printed wiring substrate 25, having the shaft 22 passing through it, is provided between the rotor 24 and the bearings 23B.
The lower case 29 is provided with the opening 32 at the center of the bottom and the drawing duct 31 which passes through the side. The lower case 29 is also provided with an edge portion which extends outwardly at the upper side, and the openings 30A, 30B, 30C and 30D are provided at equal intervals at the edge portion.
On the upper surface of the printed wiring substrate 25, a circuit conductor pattern for the circuit structure shown in FIG. 6 or FIG. 8 is formed, the peripheral circuits 26 including the sensor amplifier 8 and the speed signal formation circuit 9, together with the Hall device sensors 7A, 7B and 7C and the one-chip three-phase inverter 15, are mounted thereon, and the terminal of the circuit conductor pattern is guided to the lower surface of the printed wiring substrate 25, with the lead wires 28 connected to the terminal.
The rotor 24, together with the shaft 22, is inserted into the inside of the stator core 18, and the bearings 23A are fixed to the upper surface of the inside of the upper case 17. When the rotor 24 is positioned, the upper portion of the shaft 22 extends to the outside through the opening 20 of the upper case 17. The printed wiring substrate 25 is fixed to the supporting members 21A, 21B and 21C which extend below the stator core 18, by the bolts 27A, 27B and 27C. The lower case 29 is fixed to the upper case 17 so as to cover the printed wiring substrate 25 and the stator core 18, by matching the bolt holes 34A, 34B, 34C and 34D at the edge portion of the upper case 17 with the openings 30A, 30B, 30C and 30D at the edge portion of the lower case 29 respectively and by fastening bolts 33 to the bolt holes 34A, 34B, 34C and 34D through the openings 30A, 30B, 30C and 30D. In this condition, the bearings 23B at the lower end of the shaft 22 are fixed within the opening 32 of the lower case 29 and the lead wires 28 are guided to the outside from the inside of the lower case 29 through the drawing duct 31.
FIG. 5 shows a partially extended view of the room air conditioner using the above-described motor as a fan motor. The room air conditioner comprises an indoor unit 35 disposed within a room, an outdoor unit 36 disposed outside the room and a pipe 37 connected between these units. A tangential floor fan 38A and a propeller fan 38B are provided in the indoor unit 35 and the outdoor unit 36 respectively. The above-described motor can be used as drive motors 39A and 39B for driving the fans 38A and 38B respectively. Usually, the three-phase inverter of the motor has about the same size as that of the motor body. However, since a one-chip three-phase inverter is incorporated in the above motor, the control section of the outdoor unit 36 can be made more compact by that amount, and the outdoor unit 36 itself can be made more compact accordingly. This also applies to the indoor unit 35.
A high-power device such as a switching transistor within the one-chip three-phase inverter is a heat service, and therefore it present a problem that the properties of the one-chip three-phase inverter are deteriorated, resulting in reduced reliability, when the one-chip three-phase inverter is heated to a high temperature. Accordingly, it is necessary to provide a heat radiation fan in the one-chip three-phase inverter.
However, as explained with reference to FIG. 4, not only the one-chip three-phase inverter 15 but also the Hall device sensors 7A, 7B and 7C and their peripheral circuits 26 are mounted in a large number on the printed wiring substrate 25, so that there has been no space to provide a heat radiation fan in the one-chip three-phase inverter 15 according to the prior art. As a result, about 20W has been the limit to the output of the motor.